Beam-expanding device

ABSTRACT

A radiation beam expander includes planar optical waveguide ( 2 ) and stripe waveguide ( 3 ) provided with a set of unit reflectors ( 4 ) that overlap its aperture, and is arranged in the plane of planar waveguide ( 2 ), within said waveguide itself or in the vicinity thereof, with providing the possibility of transition of light beams ( 6′ ), reflected by unit reflectors ( 4 ), into planar waveguide ( 2 ). Angle of inclination and arrangement of unit reflectors ( 4 ) being selected such that phase difference on the operating radiation wavelength, for any pair of beams ( 6′ ) reflected from different unit reflectors ( 4 ), is essentially multiple of 2π.

FIELD OF THE INVENTION

[0001] The invention relates to optics, and particularly concerns anoptical beam expander.

PRIOR ART

[0002] Known is an expander of an optical beam propagating in a planaroptical waveguide, which expander includes a waveguiding lens (see,e.grating. Y. Abdelrezak, Chen S. Tsai and T. Q. Vu. An Integrated OpticRF Spectrum Analyzer in ZnO—GaAs—AlAsAs Waveguide, J. LightwaveTechnology, 1990, Vol. 8, No. 12, pp. 1833-1837). The device operatessimilarly to a standard light beam collimator implemented on a volumelens. A small-size optical radiation source is positioned in focus of aplanar lens, the light from which source first is expanded due to thediffraction divergence, and then the light is collimated by the planarlens. Irrespective of a possible type of the used waveguiding lenses(Fresnel, Luneberg, geodetic lenses, etc.), this device is characterizedwith large dimensions due to both geometric sizes of lenses themselvesand a great length (not less than focal distance of lenses themselves)that is required to form a sufficiently broad and well-collimated lightbeam.

[0003] Also known is a beam expander (V. Neumann, C. W. Pitt, L. M.Walpita, Guided-wave holographic grating beam expander—fabrication andperformance, Electronics Letters, 1981, v. 17, No. 4, p. 165-166),wherein an optical beam in a waveguide is expanded using the Braggdiffraction grating. This device includes a planar optical waveguide anda beam expanding means in the waveguide plane, which means is disposedon the path of the radiation beam and is a diffraction grating having avery small spacing, grooves of which grating are implemented on theoptical planar waveguide surface at the angle that is equal to Braggangle (θ_(B)) as measured in respect of the incident light beam:

sinθ_(B) =K/2k  (1)

[0004] where K=2π/Λ, 79 is diffraction grating spacing, k=2π N/λ₀ isconstant value of propagation of light in a waveguide, λ₀ is lightwavelength in vacuum, N is effective refractive index of the guided modeof the optical waveguide.

[0005] This device is capable of providing deflection by 90 degrees of anarrow (less than 1 mm) light beam that is incident onto the diffractiongrating at Bragg angle. Width of the expanded beam can be 5-10 mm. Thewidth depends on parameters of the interacting waves and diffractiongrating. In the known device, the diffraction grating on a glasswaveguide was manufactured by the method of ion etching through aphotoresist mask illuminated by a holographic technique by superpositionof two optical beams. The grating had spacing of 0.6 μm and was 0.3 nmdeep. That approach provided the diffraction efficiency of 16% for theguided mode, with effective refractive index of 1.536 at the helium-neonlaser wavelength.

[0006] But the discussed device has the following disadvantages: thehigh divergence of the outputted light beam, which divergence isdetermined by divergence of the incident light beam having a narrowaperture, as well as the spatial inhomogeneity of the expanded beam,which inhomogeneity is caused by the technical difficulty of fabricationof diffraction gratings of sub-micron sizes on a large aperture. Thislimitation is a principal one in terms of the practical use of a beamexpander in acousto-optical (AO) devices for processing and transmittingdata, e.g. AO spectrum analyzers, tunable filters, etc. In such devices,the optical beam divergence determines such important parameter as thenumber of resolvable spots.

SUMMARY OF THE INVENTION

[0007] The invention is basically directed to the object of developingof a beam expander that will have the minimal dimensions and,simultaneously, a low divergence of the outputted optical radiation.

[0008] Said object is to be solved as follows: in a radiation beamexpander that includes a planar optical waveguide and a beam expandingmeans in the planar waveguide plane, which means is disposed on the pathof the radiation beam: according to the invention—the beam expandingmeans is implemented in the form of a stripe waveguide provided with aset of unit reflectors that overlap its aperture, and which is disposedin the planar waveguide plane either within said waveguide, or in thevicinity thereof, with providing of possibility of transition of theradiation beams, reflected by the unit reflectors, into the planarwaveguide, the inclination angle and relative position of the unitreflectors being selected such that the phase difference at theoperating radiation wavelength, for any pair of beams reflected fromdifferent unit reflectors, is essentially multiple of 2π.

[0009] To ensure an essential suppression (over 20-30 dB) of sidelobesof the outputted optical radiation's directivity pattern, the unitreflectors are advantageously implemented as having differentreflectance, whose value diminishes from the middle portion of a stripewaveguide to its ends.

[0010] In the preferable embodiment, angle of inclination of the unitreflectors with respect to the longitudinal axis of a stripe waveguideis selected to be essentially 45°.

BRIEF DESCRIPTION OF DRAWINGS

[0011] The invention is further described by examples of its particularembodiments, taken in conjunction with the accompanying drawings,wherein:

[0012]FIG. 1 shows a beam expander having a stripe waveguide in thevicinity of a planar waveguide, according to the invention, isometry;

[0013]FIG. 2 shows the same expander as of FIG. 1, wherein a stripewaveguide is disposed in a planar waveguide;

[0014]FIG. 3 shows relationship between the beam expander radiationamplitude and direction of the reflected beam in the planar waveguideplane.

BEST MODES OF EMBODIMENT

[0015] A thin layer, several micrometers thick and having refractiveindex that is greater than that of the environments (substrate andambient upper layer of, in this case, air), is implemented uponsolid-state substrate 1 (FIGS. 1, 2). Said layer is planar opticalwaveguide 2, wherein a light beam can propagate within said layer withvery low losses (less than 1 dB/cm). The number of the guided(waveguiding) waves (modes) supported by this structure, and the spatialdistribution of their guided modes are determined by the profile ofchange of refractive index in depth.

[0016] In the plane of planar optical waveguide 2, within said waveguide(FIG. 2) or near it (FIG. 1), stripe optical waveguide 3 is implemented.As compared with refractive index of planar waveguide 2, stripe opticalwaveguide 3 has the higher refractive index value not only in depth, butalso in the transverse direction of the structure. Thus said waveguideis able to maintain propagation of a narrow and non-divergent opticalbeam along its axis in the region of an increased refractive indexvalue. The stripe waveguide 3 is oriented in parallel to the lateraledge of planar waveguide 2 and is a local region on, or over thesolid-state surface in the form of a thin film, several microns or tensmicrons wide, having refractive index exceeding that of its environment.Waveguides 2 and 3 can be manufactured using the following techniques:diffusion of metals, proton exchange from salt melts, sputtering of thesubstances that have a greater refractive index that that of substrate1, modification of the surface layer properties by radiation, e.g. byelectrons and/or photons, epitaxy from gaseous or liquid phase, etc.

[0017] Within the region occupied by stripe optical waveguide 3,provided is a set of inclined unit linear reflectors 4 that overlap theaperture of stripe optical waveguide 3. The optical beam to be expandedis inputted into stripe optical waveguide 3 via input 5, for example,via polished edge. From input 5 the light further passes through stripewaveguide 3 and, encountering unit reflectors 4, is splits into a largenumber of coherent light beams that transit into planar opticalwaveguide 2 and form the slightly-diverging expanded output beam 6 thatis directed to exit 7. When stripe optical waveguide 3 is implemented inthe nearest vicinity—at distance “a”—from planar optical waveguide 2(FIG. 1), reflected light beams 6′ are tunnelled through the (distance“a”) that separates them and has a lower refractive index. For thepurpose to reduce the optical losses of the device, width of said regionmust be sufficiently great so that the incident optical fields of modesof stripe waveguide 3 would not reach planar waveguide 2 (i.e. theradiation attenuation must be eliminated). On the other hand, saiddistance “a” must be sufficiently small to let tunnelling of thewaveguiding mode therethrough. In this aspect, the compromise is thevalue of separating region “a” that is equal, in an order of magnitude,to width of stripe optical waveguide 3 itself, i.e. about 5-20 μm. Whenstripe optical waveguide 3 is implemented directly in planar opticalwaveguide 2 (see FIG. 2), the reflected light beams simply passing fromone waveguide to another, thereby intersecting the separating boundarytherebetween with negligible losses.

[0018] To provide the maximum expansion of the optical beam, inclinationof reflectors 4 is selected such that the reflected beam is deviated atthe angle approximate to the right angle (i.e. unit reflectors 4 must beinclined at the angle about 45 degrees with respect to the longitudinalaxis of the stripe optical waveguide). However, generally theinclination may be arbitrary one, basing on the structure of theparticular device. By varying reflectance R and number “M” of reflectors4, both the angular divergence of outputted beam 6 and the sidelobelevel can be changed. In particular, FIG. 3 shows angular spectra ofbeam expanders having aperture of 0.7 cm for two typical cases. Curve 7corresponds to the constant reflectance for all reflectors 4, and curve8 corresponds to the case when unit reflectors 4 have differentreflectances whose value decreases from the middle portion of stripeoptical waveguide 3 towards its ends; in this case—in accordance withthe truncated Gaussian function. The form of the angular spectrum, inparticular its angular divergence and sidelobe level, are depend onparticular type of the function according to which reflectance ofreflectors 4 will be changing. But the common trend of decrease of thesidelobe levels will be maintained for any type of the reflectance valuedecrease function—from the centre part of stripe waveguide 3 to itsends.

[0019] Reflectors 4 can be in the form of local regions as narrow (about0.2-2 μm) strip having altered optical properties, for example due tothe proton exchange, ion implantation, etc., and also shaped as groovesor steps (about 10-1000 nm high) made of the same or other material onthe optical waveguide 3 surface. Reflectance R of a unit reflector 4 isin general 0.005÷0.0001 and can be controlled by selection of amanufacture process and reflector 4 geometry. Number of reflectors 4must be sufficiently great (in general, product R*M is over 2, i.e. M isapproximately 500÷1000), so that good collimating properties (a narrowdirectivity and an high level of suppression of sidelobes in angularspace) and the high efficiency of transformation from a narrow beam intoa broad one could be obtained. The distance “b” between unit reflectors4 is usually comparable with width of optical waveguide 3 (about 5÷20μm).

[0020] The device operates as follows. A narrow optical beam is inputtedinto stripe optical waveguide 3 through input 5; said beam at each ofunit reflectors 4 is divided into two beams. One beam (having asignificantly smaller intensity) is reflected and transits from stripeoptical waveguide 3 into planar optical waveguide 2, and the other beam(of a greater intensity, slighty less intense than the incident one)passes through stripe optical waveguide 3 to the next unit reflector 4,whereon it is divided again into two beams, and so on. All reflectedbeams 6′ are summed taking into account the optical phase shift causedby a delay of the light beam in the interval between adjacent reflectors4. Owing to the fact that on the operating light wavelength the phasedifference for beams 6′, reflected from different unit reflectors 4 andtransiting from stripe optical waveguide 3 into planar optical waveguide2, is selected to be multiple of 2π, then all reflected beams 6′ aresummed coherently. The resulting light beam 6 has a greater width(hundreds and thousands times greater than the inputted one) and a lowdivergence of the outputted optical radiation, caused by constancy ofthe phase front of the optical wave in the transverse direction ofstripe waveguide 3, as well as caused by a strictly predeterminedinclination and position of a great number of unit reflectors 4.

[0021] The basic difference between the proposed beam expander and thebeam expander having a holographic diffraction grating should beemphasized. In the latter device, the light is incident on theholographic diffraction grating at Bragg angle and is deviated in thefirst diffraction order. The reason is that its is necessary to providea diffraction grating having a very small spacing (fractions of micron)that is fabricated by a holographic method in view of the technologicaldifficulties. The device according to the invention does not use theeffect of Bragg diffraction, but utilizes interference of light beamsformed by a great number of reflectors. The interference proceeds inhigh interference orders, i.e. the phase difference between adjacentbeams exceeds number 2π by many times (about 10). This allows to providea much greater spacing (5-10 μm) of arrangement of unit reflectors 4,than the spacing of grooves of the diffraction grating of theholographic beam expander. For this reason, the claimed device is moremanufacturable. The different physical nature of the two compared typesof beam expanders results in that they are described using differentterms, and have different physical properties. In particular, fortypical structures of these devices, inclination and shape of thestripes will be basically different. A holographic beam expanderrequires the sinusoidal surface corrugation whose grooves are inclinedat Bragg angle with respect to the incident beam. The claimed devicerequires the narrow vertical ruling having the groove inclination thatis determined by the ruling arrangement spacing, but in any case it isseveral times (about 10 times) greater than Bragg angle designed for theoperating light wavelength.

[0022] Operation of the claimed beam expander can be illustrated by anexample of description of behaviour of the reflected wave's opticalfield in the form of its angular spectrum in the waveguide plane. Theresulting angular spectrum U(p) radiated by the beam expander isdescribed as follows. For simplicity, below follows description of thetransverse distribution of electric field of the guided (waveguiding)mode as exp(−(y/w₀)²) is chosen, where w₀ is effective width of stripewaveguide, y is transverse coordinate (in the plane of planar waveguide2). Each reflector 4 has width 2w, and described by constant reflectanceR and phase shift kx_(m), where x_(m) is coordinate of m-th reflector.Assuming that reflectors 4 are arranged strictly periodically with pitchd:

x _(m) =dm  (2)

[0023] Then U(p) can be expressed as follows: $\begin{matrix}{{{U(p)} = {\sum\limits_{m = 1}^{M}\quad {{u_{o}(p)}r\quad t^{m - 1}{\exp \left( {{- }\quad k\quad p\quad x_{m}} \right)}}}},} & (3)\end{matrix}$

[0024] where r=R^(1/2), t=(T)^(1/2), T=1−R, p is sine of observationangle as measured with respect to the axis that corresponds to directionof the beam reflected from a unit reflector; u₀(p) is angular spectrumradiated by a unit reflector. $\begin{matrix}{{u_{o}(p)} = {C{\int_{- w}^{w}{\exp\left( {{{{- }\quad k\quad p\quad x} - {\left( {x/w_{o}} \right)^{2}\quad {x}}},} \right.}}}} & (4)\end{matrix}$

[0025] where C is normalization constant.

[0026] For simplicity, it is assumed that w/w₀ ratio is much more thanunity, then the following expressions can be derived:

u ₀(p)=Cw ₀(π)^(1/2) exp(−(kwp/2)²,  (5)

u(p)=(π)^(1/2) w ₀exp(−(kwp/2)²)r(1−t ^((M−1)) exp(−ikpdM)/(1−texp(−ikpd))  (6)

[0027] Angular distribution of the expanded beam intensity will berepresented as follows.

1(p)=/u(p)/² =C ²π exp(−(kwp)²/2×

×((1−t ^(M−1))²+4t ^(M−1) sin²(kd(1−p)M−1)/2)/((1−t)²+4tsin²(1−p)/2))  (7)

[0028] The radiated spectrum of the beam expander when λ₀=1.54 μm, isshown in FIG. 3 for constant reflectance R=0.002 (curve 7). It has avery narrow peak about 0.0001 radian wide. In analysis it was assumedthat N=2.2, unit reflectors 4 are arranged strictly periodically withspacing of d=7 cm, number of reflectors is M=1000, total length of thereflectors' structure dM=0.7 μm, effective width of stripe opticalwaveguide w₀=10 μm. Curve 8 corresponds to the case when unit reflectors4 are implemented with alternating (different) reflectance whose valuedecreases from the middle portion of stripe optical waveguide 3 to itsends; in this case—according to truncated Gaussian function: r(i)=rexp[5((i−500)/1000)²]. Curve 8 is derived by numerical integration ofrelationship (3), taking into account that r and t depend on serialnumber of reflector (i). It is obvious that owing to implementation ofunit reflectors 4 with alternating reflectance whose value decreasesfrom the middle portion to ends of a stripe optical waveguide, anessential suppression (over 20 dB) of sidelobes of directivity patternof the outputted optical radiation can be attained.

[0029] Position of the angular spectrum maximum of the claimed beamexpander is determined as follows:

kd(1−p)=2π m _(λ)  (8)

[0030] where m_(λ) is interference order (integer). This maximumcorresponds to the interference order (m_(λ)) for which the propagationdirection is very proximate to the mirror reflection (p=0) from the unitreflectors. For our case m_(λ)=10 when λ₀=1.54 μm.

[0031] Thus arrangement of reflectors 4 is selected according toequation (8) when p=0. This complies with the condition that at theoperating optical radiation wavelength the direction of propagation ofone of interference orders (m_(λ)) and that of the mirror-reflectedbeams practically coincide. In other words it means that inclinationangle and position of unit reflectors 4 are selected such that, on theoperating light wavelength, phase difference for the beams reflectedfrom different unit reflectors is essentially multiple of 2π.

[0032] Further, according to (8), directivity pattern of expanded beamshifts as a whole (scans) when the light wavelength change, as follows:

p=(λ_(m)−λ)/λ_(m)  (9)

[0033] where λ_(m)dN/m_(λ).

[0034] The radiation beam expander according to the invention hasminimal dimensions (in our example: the operating field is equal to0.002×0.7 cm²) and, simultaneously, a low divergence (about 0.0001radian) of the outputted optical radiation whose directivity patternscans as the optical radiation wavelength varies.

[0035] Industrial Applicability

[0036] The radiation beam expander according to the invention can besuitably used as collimating or selecting elements in various integratedoptical circuits. Further, the property of the beam expander to changethe radiation direction as the light wavelength varies, can be of use indesigning optical tunable filters for the wavelength-divisionmultiplexing (WDM) systems applied in fiber-optic communication. Thebeam expander can be fabricated according to the known manufacturingtechniques developed for the integrated optics and microelectronicsdevices.

1. A radiation beam expander, comprising planar optical waveguide (2) and a beam expanding means in the plane of planar waveguide (2), which means is arranged on the path of radiation beam (6), characterized in that the beam expanding means is implemented in the form of stripe waveguide (3) provided with a set of unit reflectors (4) that overlap its aperture, and is arranged in the plane of planar waveguide (2), within said waveguide itself or in the vicinity thereof, with providing the possibility of passing of radiation beams (6′), reflected by unit reflectors (4), into planar waveguide (2), angle of inclination and arrangement of unit reflectors (4) being selected such that phase difference on the operating radiation wavelength, for any pair of beams (6′) reflected from different unit reflectors (4), is essentially multiple of 2π.
 2. The beam expander as claimed in claim 1, characterized in that unit reflectors (4) have different reflectance whose value decreases from the middle portion of strip waveguide (3) towards its ends.
 3. The beam expander as claimed in claims 1 or 2, characterised in that angle of inclination of unit reflectors (4) with respect to longitudinal axis of stripe waveguide (2) is essentially 45°. 